Optical bay fit like a slightly shrunken glove
I was trying to give my 2008 iMac a few more years by adding an SSD and giving it a home-brew Fusion Drive.
My first attempt failed due to my personal stupidity. My reaction to the $59.95 price for the optical=>SATA enclosure was "That has GOT to be cheaper elsewhere." So I found and ordered one for $12 on Amazon, along with an SSD and a USB enclosure for the optical drive.
After 45 minutes of disassembly, I discovered that my iMac had a PATA optical drive, and thus neither enclosure would fit. I put it all back together and ordered the real thing from iFixit.
After the second round of disassembly, I discovered that the screw holes in the plastic bracket didn't quite line up with the ones in the optical drive=>SSD enclosure. To make it fit, I had to take the rubber grommets out of the bracket and replace them with felt "washers" so the screws could go to one side of the large grometted holes. Sorry, no picture - could not coerce camera into macro closeup mode.
After that, the only speed bump was creating the Fusion Drive. All the various recipes and incantations I saw on the net implied that you could boot off a drive's recovery partition and use the command line to merge that drive with the SSD.
NOPE. I had to dredge up another external drive, install Mountain Lion on that, and boot off it to get a system that would let me merge the drives and restore from Time Machine.
The last step was somewhat frustrating. The machine felt slower than usual for the first day or so (!). This is because it had to re-Spotlight-index completely - I was getting really depressed until I figured that out.
Once that drag on the system went away, it definitely became snappier. I definitely recommend home brew Fusion Drives.
1. Use iFixit's system identifier. Yes, lifting up your iMac is a pain, but so is sending parts back and disassembling yout iMac twice.
2. Sometimes not even iFixit's parts fit perfectly the first time. Be prepared to cut-to-fit at times.